Under DOS and Windows, a message is normally stored on disk as firstline\015\012secondline\015\012...\015\012lastline.
Under UNIX, a message is normally stored on disk as firstline\012secondline\012...\012lastline.
In practice, it is very common for the last line to be empty. A partial line means a nonempty last line. Beware that some text-processing utilities ignore partial lines.
For example, the three-line message
This is a test. ... Three dotsis stored under UNIX as "This is a test.\012... Three dots\012" and under DOS as "This is a test.\015\012... Three dots\015\012".